The present disclosure generally relates to electrochemical sensors. More particularly, it relates to sensors for determining the electrochemical corrosion potential (ECP) of metal components in liquids at high temperatures and pressures, as well as to methods of using the sensors.
Many areas of industry, such as the power generation industry, employ metal structural components that are exposed to liquids at high temperatures and pressures. Examples of systems in which equipment is designed for such exposure include nuclear reactors, including the boiling water and pressurized water type reactors, fossil fuel systems, and geothermal systems. In a boiling water nuclear reactor, for example, water and steam are channeled through various conduits formed of iron (Fe) and nickel (Ni) based alloys. Normal water chemistry conditions produced by radiolysis in-core, include highly oxidizing species, such as oxygen and hydrogen peroxide, which may lead to high electrochemical corrosion potential (ECP) and, eventually, intergranular stress corrosion cracking of the stainless steel.
Intergranular stress corrosion cracking can be mitigated by lowering the concentrations of oxidizing species in the reactor water, which results in low ECPs. The hydrogen is added to the feed water of the reactor to reduce the dissolved oxidant concentration and lower the ECP below a specific value at which intergranular stress corrosion cracking susceptibility is significantly reduced. When hydrogen water chemistry is practiced in a boiling water reactor, the electrochemical corrosion potential of the stainless steel components decreases from a positive value, generally about 0.050 to about 0.200 volts (V), based on a standard hydrogen electrode (SHE) as a reference, under normal water chemistry to a value of less than about −0.230 V (SHE). There is considerable evidence that when the electrochemical corrosion potential is below this negative value, intergranular stress corrosion cracking of stainless steel can be mitigated and the intergranular stress corrosion cracking initiation can be prevented.
Thus, considerable efforts have been made in the past decade to develop reliable electrochemical corrosion potential sensors to be used as reference electrodes to determine the electrochemical corrosion potential of operating surfaces. These sensors have been used in boiling water reactors worldwide, which has enabled the determination of the optimum feedwater hydrogen injection rate required to achieve electrochemical corrosion potential of reactor internal surfaces and piping below the desired negative value.
Various forms of ECP sensors are used for measuring ECPs in nuclear reactors and other systems. However, these sensors are subject to different problems that limit their useful lives. For a nuclear reactor, for example, the useful life of a sensor should cover the duration of at least a single fuel cycle, which is in the range of about 18 months to about 24 months in the United States. Experience in actual nuclear reactors has demonstrated sensor failure in a shorter duration due to various causes. An ECP sensor experiences a severe operating environment in view of the high temperature of water, well exceeding 280 degrees Celsius (° C.), and relatively high flow rates thereof, up to and exceeding several meters per second (m/s).
One type of ECP sensor includes a ceramic probe in the form of a zirconia tube brazed to a metal alloy tube. Since the ceramic probe and metal tube have different coefficients of thermal expansion, they are subject to thermal shock during high temperature operation which can lead to cracking of the braze joint. The braze material is also subject to corrosion during operation. Both problems limit the useful life of the sensor, since failure of the brazed joint causes water leakage inside the sensor and failure thereof.
Accordingly, a continual need exists for improved ECP sensors with increased operating lifetimes.